1. Field of the Invention
The present invention relates to a throttle device for a paint having a high viscosity, and used in an airless paint supply system.
2. Description of the Related Art
In a conventional airless paint supply system, a back pressure is applied to a high viscosity paint in a paint supply line, to allow a spray gun to emit paint at a high pressure, and for this purpose, a throttle valve is used. Two conventional types of throttle valves are used for a high viscosity paint; i.e., one having a high pressure regulator having a movable valve means, and another having an orifice tube having a constant small opening area. Currently, the orifice tube type is most widely used, since the high pressure regulator type suffers from a problem of the endurance of the movable parts thereof. A typical example of such an orifice tube type valve is shown, for example, in Japanese Unexamined Utility Model Publication No. 62-6594.
The orifice tube provides a flow resistance, to cause a loss of pressure in the flow of paint flowing therethrough, and thus can apply a back pressure to paint in the paint supply system. To apply a high back pressure, the orifice passage of the orifice tube must be very long and have a very small diameter. For example, the orifice passage must be 1 mm in diameter and 42 mm in length to provide a back pressure of 100 kgf/cm.sup.2. If the diameter is enlarged the length must be increased accordingly. To generate a similar loss of pressure, for example, the length must be 750 mm, if the diameter is 2 mm; and must be 4000 mm, if the diameter is 3 mm. It becomes very difficult to use such an excessively long orifice tube.
Therefore, in practice, an orifice passage having a smaller diameter is used, and in general, an orifice passage as described above that is 1 mm in diameter and 42 mm in length is used. But the paint often includes dust or foreign matter, and thus the orifice tube is often clogged; and when the orifice tube becomes clogged, it is difficult to clean same since the orifice passage is very narrow, as stated above. Also, the paint flow velocity becomes higher in such a narrow orifice, so that if the paint including an abrasive material, such as an interlayer chipping resistant paint, is used, wear of the inner surface of the orifice passage occurs and thus the opening area is enlarged.